1. Field of the Invention
This invention relates to apparatus for analyzing gases and vapors to determine the presence of compounds and, more particularly, to apparatus for improving the range and sensitivity of electron-capture ionization detectors which are used in such analyzing apparatus.
2. Description of the Prior Art
In the copending application of Conrad S. Josias, et al., Ser. No. 835,290, filed May 29, 1969, and assigned to the assignee of the present invention, a gas detector and analyzer was described which utilized an electron-capture ionization detector to signal the presence of very low concentrations of different chemical compounds in an environment. That application cited and relied upon a prior patent to James E. Lovelock, U.S. Pat. No. 3,247,375, which taught an electron-capture ionization detector and circuits which made such a device a useful tool for analysis.
Recently, Dr. James E. Lovelock delivered a paper entitled "Analysis by Gas Phase Electron Absorption" at the Seventh International Symposium on Gas Chromotography Discussion Group of the Institute of Petroleum held at the Falkoner Centret, Copenhagen, Denmark, from June 25 to June 28, 1968. The paper was subsequently published by the Institute of Petroleum of London, W1, Great Britain in 1969 as part of a volume entitled "Gas Chromotography, 1968," edited by C. L. A. Harbourn.
The Lovelock paper described in some detail the history of the electron-capture detector noting that electron absorption was a technique almost entirely dependent upon gas chromatography for its existence, the "electron-capture" detector being so sensitive that it could function efficiently only with pure vapors greatly diluted in a clean stream of carrier gas emerging from a chromatograph column. That article is considered supplementary to and cumulative of Dr. Lovelock's prior papers, including the article "Ionization Methods for the Analysis of Gases and Vapors," published at page 162 in the Feb., 1961, issue of "Analytical Chemistry," Volume 33, No. 2, and a subsequent paper entitled "Electron Absorption Detectors and the Technique for their Use in Quantitative and Qualitative Analysis by Gas Chromatography," published at page 474 of Analytical Chemistry, Volume 35, No. 4, of April 1963.
In the Gas Chromatography 1968 article, Lovelock also described the chemical and physical basis for the operation of the electron-capture detector and discussed the parameters that were important in the construction of such a detector. At page 102, Lovelock discussed the methods of operating such electron-capture detectors. A severe drawback of the earliest versions was the limited dynamic range of such detectors. The dc method then employed applies a fixed potential difference between the electrodes of the detector. The detector is subjected to a stream of inert carrier gas which does not itself absorb electrons. The potential is adjusted to a value sufficient to collect all of the electrons liberated from the carrier gas by a radiation source which ionizes the gas.
An electron-absorbing vapor introduced into the gas stream collects the free electrons to produce negative molecular ions which then recombine with the positive ions resulting from ionizing radiation. The change in current flow attributable to the presence of electron-capturing compounds is determined. If the decrease of current flow is measurable, then a quantitative indication of the electron-absorbing compound can be achieved.
Alternatively, the potential can be increased to a value that maintains the current flow at a constant value and the change of potential would also represent a measure of the quantity of electron-absorbing compound present. Yet other methods utilize higher potentials, but generally, such higher potentials result in a nonlinear response to vapor concentration.
As described by Lovelock in the 1963 Analytical Chemistry paper, supra, a pulsed sampling technique can be employed involving the use of brief pulses of potential, at relatively infrequent intervals. Lovelock suggested a 50-volt pulse of 0.5-microseconds duration, at intervals of approximately 100 microseconds. This pulsed sampling procedure enjoyed several advantages in that:
1. For most of the time, no field is applied to the detector so that free electrons are in thermal equilibrium with gas molecules;
2. The sampling pulse is so brief that no significant movement of negative ions occurs, avoiding inaccuracies due to space-charge effects or the collection of negative ions at the anode;
3. A pulse amplitude of 30 volts is sufficient to collect all of the electrons;
4. The ultimate sensitivity is increased since the time for encounter between absorbing molecules and electrons is extended to the point where natural recombination limits any further increase in sensitivity; and
5. Except for those compounds whose absorption cross-sections increase greatly with small increases in energy, and for which sensitivity improves only in dc systems, the pulse method is much more reliable, and in general, sensitivity is improved.
In the copending Josias, et al. application, the pulsed sampling technique as described by Lovelock was modified. A highly-stable pulse source, for example, a crystal-controlled oscillator whose frequency stability exceeds one part in 10.sup.8, was provided. The magnitude of the pulses was reduced to approximately 30 volts, and the pulse duration was extended to 3 microseconds. These pulses were repeated at 100-microsecond intervals. It appeared that the lower-voltage pulses of longer duration also adequately swept all of the electrons from the ionization detector and provided a current which, when averaged, could be used to signal concentration.
In the Gas Chromatography article, Lovelock, at pages 102 and 103, disclosed yet other improved pulse methods for increasing the dynamic range of the detector. Detectors were described in which a signal for measurement was not produced directly. Rather, the detector serves as a sensor to indicate a departure from a steady-state condition.
One circuit was disclosed in which the output of an electrometer amplifier was fed back to a pulse generator where it was compared to a reference current. The result of the comparison was used to set the pulse interval. The output of such a system was not a current to a recorder, but was a digital or frequency signal.